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What is this? Answered

I was cleaning my closet and I found this. a used hand warmer. Being curious, I opened this sucker up and found black powder with what looks like a small bit of silver powder. My guess is its iron, magnesium, and salt.

3 Replies

I think it is iron oxides(FeO, Fe2O3, etc) and salt (NaCl). I remember a few years back reading a patent for one of these things, and basically it was very small particles of iron metal coated with salt. Opening the packet to air containing even minute amounts of moisture is sufficient to start oxidizing the iron particles, and this is what produces the heat. That is to say, iron oxides are what's left over once the reaction is complete.